Relay-Operated Switch for Over Voltage and Under Voltage

The circuit was designed to provide a voltage over and under the specified value with the use of a switch that will be operated by a relay.

• LM741 – a general purpose single operational amplifier with features such as offset null, compensated internal frequency, voltage range with high input, good stability of temperature, and protected from short circuit
• BC214 – a complementary silicon planar epitaxial transistor used in AF small signal drivers and amplifiers as well as for low noise preamplifier applications due to its feature of good linearity of DC current gain

The figure shows a switch containing a relay that gives a precise DC with under voltage value. A voltage comparator is based on the operation of op-amp LM741where it switches its output to which is lower. The test voltage is applied to pin 3 while the reference voltage is applied to pin 2. In the event that an excess of 5 V is present, the circuit can be made to trip using a suitable value for the resistor R1. A trip voltage represents the power supply voltage above which the protection circuit will activate and will allow the system to function with normal power supply. The presence the Zener diodes is to regulate the amount of voltage flowing across both circuits.

The second figure points out the over voltage relay switch. In excess of 10 mV, the circuit can be made to trip at any preset voltage. If the trip values falling in the range of 10 mV to 3 V are required, the input voltage can be connected directly to pin 2. In the event that the voltage is in excess of 3 V, a suitable value of resistor is necessary to keep in a suitable level the voltage drive of pin 2. By transferring the pin 2 and pin 3 connections of the op-amp, the over voltage switch can be converted to under voltage switch. This over voltage circuit can rectify the AC input signal which can then be applied as an AC voltage switch afterwards.

The relay of over voltage and under voltage circuit can be used for protection of transformers, motors, and generators as the relay is generally used as a three-phase relay. This is the reason why the switch setting values and positions have to be altered for obtaining the required values. Another important application of this circuit is in the industrial power distribution network where the electromagnetic connection to the network turns the circuit breaker off when the voltage fluctuates to the load range of +10%. They are considered very useful for the protection of industrial electrical installations.

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